Solid-state lasers have found applications in all areas where high peak power or high continuous power are required. Examples include material processing (cutting, drilling, welding, marking, heat treating, etc), semiconductor fabrication (wafer cutting, IC trimming), graphic arts (printing, copying), biotechnology instrumentations (proteomics, DNA sequencing, flow cytometry), medical applications (diagnosis, therapy, micro-surgery), military applications (range finding, target designation), entertainment (laser TV, DVD), and scientific research, to mention a few.
Unlike semiconductor or diode lasers, which are almost always pumped electrically, solid-state lasers based on active ions doped in crystals or glasses are optically pumped. One of the key components of a solid-state laser therefore is an efficient and low-cost light source to provide the optical pumping.
Such optical pumping of solid-state lasers requires the efficient conversion of electrical energy into optical radiation, and an efficient coupling between the generated high-radiation fluxes and the solid-state laser active (gain) medium. Efficient coupling requires a close match between the output spectrum of the pumping source and the characteristic absorption bands of the particular gain medium employed. To maximize the laser output and minimize thermal effects, precise spatial overlap and uniform absorption of pumped photons over the laser mode volume is important.
Flash lamps, arc lamps, laser diodes, and some nonelectric light sources have been employed to pump solid-state lasers over the past years. All of these pumping sources have serious limitations and drawbacks, however.
Historically, flash lamps have been widely utilized for pumping solid-state lasers partly because of their high conversion efficiency. However, due to their non-monochromatic output, the coupling efficiency is generally low. Increasing the flashlamp's filling pressure could improve the conversion efficiency, however, this would require higher trigger voltage and the simmer current would be more difficult to establish and maintain. The flashlamp's low coupling efficiency causes a large amount of heat to be generated during the pumping, which limits repetition rates of solid-state lasers pumped by flash lamps. Additionally, the excessive heating leads to undesirable thermal effects such as thermal birefringence, thermal lensing, and even thermal damage. Finally, flash lamps typically exhibit short operating lifetimes. Consequently, their frequent replacement is necessary.
In sharp contrast to flash lamps, semiconductor diode lasers produce characteristically narrow emissions, which may be advantageously matched to the absorption peak of a laser's active medium, resulting in a high coupling efficiency. Unfortunately however, semiconductor lasers are effectively low-peak-power devices and as such, are not applicable to high peak power pulsed mode operations and can be easily damaged by electrostatic discharge or current spikes. Moreover, the available output wavelengths of diode lasers are limited. Consequently, some wavelengths in the visible spectrum that have important applications—such as pumping vibronic crystal lasers—are not available with diode lasers. Finally, diode lasers have a relatively short lifetime of only 5,000 to 10,000 hours and their cost is relatively high.
Previously designed solid-state lasers side-pumped by diode lasers required that their temperature be strictly controlled and were much less efficient than desired. When used as a laser pump, the light output from a diode laser is directed to an end or a side of the laser's active medium by lenses, optical fibers, mirrors or lens ducts. And in spite of its high operating efficiency, diode laser pumping often does not operate over desirable temperature ranges unless inefficient and oftentimes cumbersome temperature control is used.
Typically, side-pumped lasers use cylindrical rods and thus do not exhibit efficient mode-pump overlap, which is particularly problematic for high power scaling. In a side-pumped configuration, low dopant percentage must be used to avoid absorption of pumping energy concentrated near the surface of the laser medium, which may lead to poor overlap between the laser mode and the pumped volume and therefore, hot spots inside the gain medium which results further in the degradation of the quality of the laser beam.
Still other attempts were made to pump solid-state lasers with other semiconductor sources, in particular, incoherent monochromatic light sources such as the high-intensity Amplified Spontaneous Emissions (ASE) from rare-earth-doped fluoride, telluride and silica fibers, ASE from super-luminescent diodes, spontaneous emission from light emitting diodes (LEDs), and incoherent or partially coherent emission from vertical cavity surface emitting laser (VCSEL) arrays. Among them, LEDs and VCSELs are of particular interest, because their spectral bandwidths may suitably match the absorption spectrum of the lasing medium. In addition, high power LEDs and VCSELs offer some particularly important wavelength ranges—where conventional high power edge emitting laser diodes are unavailable. And unlike a flashlamp, which runs with an expensive high-voltage power supply of a large footprint, an LED can be driven easily with a low-voltage power supply.
As an initial example, in U.S. Pat. No. 3,997,853, Morris, et al. described that single crystals of chromium-doped beryllium aluminate were capable of operating at the room temperature and being pumped by a variety of optical pumps including incoherent semiconductor diode emitters such as gallium arsenide and gallium phosphide.
In a paper published in Journal of Applied Physics, Vol. 45, No. 3, March 1974, Farmer and Kiang described the results of LED-pumped neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers. In particular, they described a solid-glass half-cylinder used for index matching and as a pumping cavity reflector for their Nd:YAG laser.
A LiNdP4O12 laser pumped with an AlxGa1-xAs electroluminescent diode was described by Saruwatara et al. in a paper published in Applied Physics Letters, Vol. 27, No. 12 on 15 Dec. 1975. Therein, the authors disclosed that using an incoherent AlxGa1-xAs light-emitting diode as a pump source, pulsed laser oscillation was observed in a LiNdP4O12 crystal cooled to −35° C.
Additional Nd:YAG investigations were performed and described by Stone, Burrus, and Dentai in a paper appeared in Applied Physics Letters, Vol. 29, No. 1, 1 Jul. 1976. The authors achieved CW laser action using single-crystal Nd:YAG fibers end-pumped by a single high-radiance LED.
In a paper appeared in Applied Physics Letters, 33(4), 15 Aug. 1978, Budin et al. described a laser made of a 1-cm-long Nd0.75La0.25P5O14 crystal with bonded mirrors, side pumped with two arrays of light-emitting AL0.1Ga0.9As planar diodes in a double cylindrical-elliptical cavity. Pulsed operation (0.6 ms) at −3.5° C. and CW lasing at −49° C. were obtained, respectively, with LED current densities of 700 and 270 A/cm2.
More recently, a “Miniature fabry-perot laser structure” was described in United States Patent Application No. 20020071456. Described therein, a laser structure consisting of an unguided uniform gain element and two mirrors forming a Fabry-Perot cavity was coupled to an LED pump.
Despite limited progress, prior art attempts at pumping solid state lasers with LED's typically followed paths taken by diode laser pumped devices, and consequently never realized potential benefits associated with LEDs. Our invention advantageously addresses these deficiencies.